1. Field of the Invention
The invention relates to an exhaust system of an internal combustion engine.
2. Description of the Related Art
In an exhaust system of an internal combustion engine, such as a V-type engine, having a plurality of cylinders, all of the cylinders are divided into two cylinder groups, each of which is provided with an exhaust manifold, and an exhaust passage is provided for each of the cylinder groups such that the exhaust passage extends from the corresponding exhaust manifold to an exhaust merging point at which streams of exhaust gas from the respective exhaust passages join together. In the exhaust system of this type, it has been proposed that a turbine of a turbocharger and a catalytic converter downstream of the turbine are disposed only in one of the exhaust passages, and this exhaust passage communicates at a junction point upstream of the turbine with a junction point of the other exhaust passage, while a control valve is disposed downstream of the junction point of the other exhaust passage. An example of the thus constructed exhaust system is disclosed in, for example, JP-B2-1-27246.
In the arrangement as described above, the control valve is used as a wastegate valve of the turbocharger, and the control valve is closed during low-load engine operation in which a small amount of exhaust gas is produced, so that the exhaust gas of all of the cylinders passes the turbine. During high-load engine operation in which a large amount of exhaust gas is produced, the control valve is opened so that the exhaust gas of the other cylinder group passes the other exhaust passage, and only the exhaust gas of the above-indicated one cylinder group passes the turbine. In this manner, the boost pressure established by the turbocharger is constantly controlled to a set value or its vicinity.
In the meantime, during startup of the engine, it is necessary to warm up a catalytic converter early so as to activate a catalyst carried thereon, thereby to start treatment of exhaust gas at an early stage. In the exhaust system as described above, the control valve is closed during startup of the engine so that the exhaust gas of all of the cylinders passes the catalytic converter. However, the exhaust gas passes the turbine of the turbocharger before flowing into the catalytic converter, and the temperature of the exhaust gas is reduced during its passage through the turbine; therefore, the catalytic converter cannot be warmed up early.
It is an object of the invention to provide an exhaust system of an internal combustion engine, which is capable of early warm-up of a catalytic converter during startup of the engine.